DTM is a GERAN (GSM EDGE Radio Access Network) feature which allows for simultaneous circuit-switched (CS) voice and packet-switched (PS) data transfers. Two sub-classes of DTM are                multi-slot DTM, where voice and data traffic is carried on separate timeslots, and        single-slot DTM, where a combination of a half-rate speech traffic channel and a half-rate packet data channel is carried on the same timeslot. This is described in the document 3GPP TS 45.002 which is incorporated by reference.        
Single-slot DTM is advantageous in congested cells, in which there are not sufficient free timeslots for a multi-slot allocation but in which Mobile Stations (MS), also referred to as user equipment and wireless devices, still require simultaneous voice and data transfer. In particular, DTM is highly useful for combined telephony and data devices in order to maintain an “always on, always connected” experience. On the other hand, a disadvantage of single-slot DTM as currently specified in GERAN is that it can only be used in combination with half-rate speech. This implies that the use of single-slot DTM will typically result in lower speech quality.